Patent literature 1 discloses a switching element. This switching element includes a semiconductor substrate having a trench provided on its upper surface. An inner surface of the trench is covered by a gate insulation layer. A gate electrode is arranged within the trench. The gate electrode is insulated from the semiconductor substrate by the gate insulation layer. An n-type source region, a p-type body region, an n-type drift region, and a p-type bottom region (p-diffused region) are provided in the semiconductor substrate. The source region is in contact with the gate insulation layer. The body region is in contact with the gate insulation layer on a lower side of the source region. The bottom region is in contact with the gate insulation layer at a bottom surface of the trench. The drift region extends from a position being in contact with a lower surface of the body region to a position being in contact with a lower surface of the bottom region. The drift region is in contact with the gate insulation layer on a lower side of the body region.
When the switching element of Patent literature 1 turns off, a depletion layer expands from an interface between the body region and the drift region. At this occasion, a depletion layer expands from an interface between the bottom region and the drift region as well. A wide range of the drift region is depleted by these depletion layers.